Last week's collapse of a coal-ash dam in Tennessee revealed nationwide gaps in how the government oversees construction of power plant impoundments and how regulators watch over the country's largest source of industrial waste, environmental groups and safety experts say.

Despite years of study - and prodding by the National Academy of Sciences and Congress - there are no federal standards for coal-ash waste disposal or dam construction.

"Flying blind without federal rules that ensure safe disposal of the largest industrial waste in the country is nothing if not foolish, dangerous, and contrary to statutory mandates and clear congressional intent," Lisa Evans, a lawyer with the group Earthjustice, told a House Natural Resources subcommittee during a June hearing.

For several years, Evans has been among the leaders of a growing group of environmental advocates who pushed for the U.S. Environmental Protection Agency to classify coal-fired power plant ash as a hazardous waste, a move that could trigger a variety of tougher limits.

On the morning Dec. 22, a dam broke at the Tennessee Valley Authority's Kingston Plant in Roane County, Tenn., about 40 miles west of Knoxville. Estimates continue to vary, but current figures from TVA say about 5.4 million cubic yards of wet ash burst from the impoundment, covering 300 acres of nearby farms, fields and rivers.

At least two leaks or "blowouts" occurred in 2003 and 2006 at the TVA facility, according to the authority's own most recent inspection report, dated February 2008.

The more recent incident was linked to "excessive seepage," caused by internal erosion, or "piping," the same sort of problems that were at least in part to blame for the 1972 Buffalo Creek disaster and the October 2000 Martin County slurry impoundment breakthrough, said Jack Spadaro, a former longtime government dam safety engineer and inspector.

"They certainly had plenty of notice," Spadaro said Monday. "TVA has really been outrageously irresponsible."

In a "Fact Sheet," authority officials said that they "made changes and repairs to improve the condition of the dike" after each previous incident.

More than half of the ash generated by coal-fired power plants in the U.S. is dumped into impoundments or landfills, according to federal studies.

Somewhat similar impoundments used for disposal of slurry, the wet waste from preparation plants that clean coal prior to it being burned, must meet strict design, construction and inspection standards set by Congress in the 1977 Surface Mining Control and Reclamation Act.

But unless power plant dams are located on an active mining site, they are not covered by those standards, said Peter Mali, a spokesman for the federal Office of Surface Mining, Control and Reclamation. Regulation of coalash dam safety has been left to state agencies, where standards, inspection staff and expertise can vary widely from state to state.

In March 2006, the National Academy of Sciences recommended close coordination between OSM and EPA in developing nationwide standards for disposal of power plant ash in surface impoundments and landfills or at mining sites.

As early as 1980, Congress had mandated that EPA decide if power plant was constituted "hazardous waste," and should be subject to stricter regulation. Not until May 2000, under the Clinton administration, did EPA issue that ruling. And then, two years later, the Bush administration essentially reversed the action, saying it would not write new hazardous waste rules for the plant ash.

Burning coal produces more than 129 million tons of "coal combustion waste," or CCW, each year. That's enough to fill one million standard railroad cars, or a train that would span the United States from New York to Los Angeles 3.5 times, according to the National Academy report.

And ironically, new rules to clean up power plant air emissions are increasing that figure, as additional pollution control equipment that generates more ash is added to aging coal-fired plants.

When coal is burned, its volume is reduced by two-thirds to four-fifths, concentrating metals and other minerals - arsenic, selenium, mercury and numerous other toxic contaminants - that remain in the ash.

During the June hearing, Evans told lawmakers that the absence of national standards has caused environmental damage at ash disposal sites across the country. EPA itself issued a report that listed 67 contaminated sites in 23 states where the ash had polluted groundwater or surface water.

"EPA admits that this is just the tip of the iceberg, because most CCW disposal sites in the U.S. are not adequately monitored," Evans said in her testimony.